Release And Multiphase Catalytic Behavior Of AAEMs During Gasification Process Of Agricultural Biomass

As a clean,efficient and abundant energy,biomass has attracted the attention of many researchers all over the world.In view of the characteristic which has a high content of alkali and alkaline earth metallic species（AAEMs）,the common agricultural residues are selected,and the release of AAEMs in different occurrence forms and their multiphase catalytic characteristics during biomass gasification have been conducted in this paper.Firstly,the release characteristic of AAEMs during biomass gasification was studied,and its release mechanism was discussed.The results showed that the main alkalis were water-soluble,especially the K element accounting for 82%,while that of alkaline earth metals were mainly ion exchange parts and acid soluble parts.Metal valence and temperature are the key factor on release of AAEMs during biomass gasification:Alkali metals were more likely to release than alkaline earth metals and the release content of alkali metals increased from 13¹6%to 50⁵8%and that of alkaline earth metals increased from11¹4%to 34⁴0%while the reaction temperature changed from 500°C to 900°C.In addition,the release degree of AAEMs in different occurrence forms has a great relation with their proportions——the higher proportion,the more release.The released AAEMs have some catalysis on the following gasification process.Then,cellulose was selected as the model compound of biomass and KCl was used for catalyst.The catalytic experiment of KCl participating in homogeneous steam-volatile and heterogeneous steam-char reforming reactions in steam gasification of cellulose was conduct.It turned out that the added steam had negligible effect on cracking/reforming of macromolecules volatiles at high temperature without KCl,whereas in the presence of KCl,it would be activated and further reacted with low reactivity macromolecules volatiles,leading to an increase in gas yield and a decrease in tar yield.At the same time,KCl can also promote water gas shift reaction and tar reforming.However,KCl has a limited effect on levoglucosan cracking/reforming in a steam-volatilization homogeneous reaction at500℃.In addition,KCl promotes heterogeneous steam-char reforming reactions and increases H₂ yield,and this promoting effect increases with the increasing temperature.The0.5wt%KCl has reached the saturated catalytic activity of the water gas shift reaction.Finally,the samples loaded with KCl and CaCl₂ were analyzed by thermogravimetric analyzer（TGA）.The results showed that the addition of AAEMs can promote the pyrolysis/gasification of biomass significantly,make the peak shape of TG/DTG curve shift to lower temperature and increase the gas yield.The apparent activation energy of pyrolysis/gasification is orderly reduced with the higher concentration of AAEMs.The activation energy of rice husk gasification is 164.46kJ/mol,and that reduces to 163.18 kJ/mol,122.27kJ/mol and 108.21kJ/mol with the introduction of 1%,5%and 10%of CaCl₂.KCl also has similar conclusions,but its effect of reducing activation energy is not as good as that of CaCl₂.